1. Field of the Invention
The present invention relates generally to an air/fuel ratio control system for an internal combustion engine for performing .lambda. control in order to maintain air/fuel ratio close to stoichiometric value. More specifically, the invention relates to an air/fuel ratio control system which can avoid influence of lowering of precision of control parameter data.
2. Description of the Background Art
As is well known, an air/fuel ratio control for an air/fuel mixture to be introduced into an engine combustion chamber is performed by modifying a basic fuel delivery amount Tp, such as basic fuel injection amount, which is generally derived on the basis of an engine revolution speed and an engine load represented by an intake air flow rate, for example, by utilizing a correction coefficient derived on the basis of an oxygen concentration dependent control parameter derived by monitoring oxygen concentration in an exhaust gas. The correction coefficient variable depending upon the oxygen concentration in the exhaust gas will be hereafter referred to as .alpha. correction coefficient. The .alpha. correction coefficient is generally derived through PI (proportional-integral) control process. In the practical process, when the air/fuel ratio varies from rich to lean across a stoichiometric value, a lean mixture proportional component P.sub.L is used. On the other hand, when the air/fuel ratio varies from lean to rich across the stoichiometric value, a rich mixture proportional component P.sub.R is used.
On the other hand, an integral component is derived by integrating an integral constant over a period while the air/fuel mixture is maintained rich or lean. The integral constant to be used while the air/fuel mixture is held rich will be hereafter referred to as rich mixture integral constant I.sub.R and while the air/fuel mixture is held lean will be hereafter referred to as lean mixture integral constant I.sub.L.
As will be appreciated, the proportional component as a fixed constant and the integral constant are variable depending upon the engine driving condition defined by engine speed, engine load and so forth.
In the normal state of the components constituting the air/fuel ratio control system and the engine is driven at substantially steady state, a threshold level corresponding to the stoichiometric value to distinguish rich mixture and lean mixture can be set at substantially mid point between a maximum value and minimum value of an oxygen concentration indicative values of an oxygen sensor in an exhaust system, which oxygen concentration indicative value cyclically fluctuates between rich mixture indicative maximum value and lean mixture indicative minimum value.
The oxygen sensor generally has a lag time to vary the oxygen concentration indicative value across the threshold level from actual variation of air/fuel ratio across the stoichiometric value. As long as the oxygen sensor operates in normal state, the lag times upon varying the air/fuel ratio from rich to lean and from lean to rich are substantially the same to one another.
Here, the oxygen sensor tends to subject secular variation for varying output characteristics and response characteristics. In case of the oxygen sensor utilizing a zirconia, secular variation is inclined to occur to lower responseability for air/fuel ratio variation from lean to rich, while maintaining responseability for air/fuel mixture variation from rich to lean. This causes shifting of the .alpha. correction coefficient to be shifted to the richer side to cause richer mixture to be supplied to the engine combustion chamber.